In recent years, there has been vigorous development of high-breakdown voltage, high-output compound semiconductor devices, making use of advantages of nitride-based compound semiconductor including high saturation electron mobility and wide band gap. The development is directed to field effect transistors such as high electron mobility transistors (HEMTs), for example. Among them, a GaN-based HEMT having an AlGaN layer as an electron supply layer attracts a lot of attention. In the GaN-based HEMT, lattice distortion occurs in the AlGaN layer due to difference in lattice constants between AlGaN and GaN, the distortion induces piezo polarization therealong, and thereby generates a high-density, two-dimensional electron gas, in the upper portion of the GaN layer laid under the AlGaN layer. This configuration ensures high output.
However, a GaN-based HEMT cannot sometimes stand for long-period use due to miniaturization. Moreover, there is a technique of vanishing the two-dimensional electron gas by forming a p-type GaN layer between the gate electrode and the electron supply layer in order to achieve normally-off operation, but current collapse is likely to happen and the characteristic is likely to be deteriorated if the p-type GaN layer is provided.    [Patent Literature 1] Japanese Laid-Open Patent Publication No. 10-12872    [Patent Literature 2] Japanese Laid-Open Patent Publication No. 2003-209246    [Patent Literature 3] Japanese Laid-Open Patent Publication No. 07-131005    [Patent Literature 4] Japanese Laid-Open Patent Publication No. 10-104985    [Patent Literature 5] Japanese Laid-Open Patent Publication No. 2010-192771